1. Field of the Invention
The present invention relates to an amplification stage in which an envelope tracking (ET) modulator is utilised to provide a power supply to an RF amplifier.
2. Description of the Related Art
With reference to FIG. 1 there is illustrated components of a known RF amplification architecture in which an envelope tracking (ET) modulator is used to provide a power supply to a radio frequency (RF) power amplifier.
As illustrated in FIG. 1, an RF power amplifier 102 receives an RF input signal to be amplified on an input line 136, and receives a modulated power supply voltage Vsupply on line 138. The RF power amplifier 102 generates an RF output signal on line 140. An example implementation of such an RF power amplifier is in mobile communication systems, with the RF output on line 140 connected to the front end of radio transmission equipment.
As illustrated in FIG. 1, an envelope signal representing the envelope of the RF input signal to be amplified is converted by a digital-to-analogue converter 126a into an analogue signal, filtered by an optional envelope filter 128a, and then provided as an input to an ET modulator 108. The output of the ET modulator 108 forms an input to an output filter 106, and a modulated supply voltage is then provided through a supply feed 104 to provide the supply voltage on line 138.
Baseband I and Q signals are converted into analogue signals via respective digital-to-analogue converters 126b and 126c, and optionally filtered through respective I and Q filters 128b and 128c. The filtered I and Q signals are provided as inputs to a vector modulator, illustrated as respective multipliers 130a and 130b and a combiner 132. The combined output of the combiner 132 forms an input to a variable gain amplifier 134, the output of which forms an input to an optional interstage surface acoustic wave (SAW) filter 112. The output of the filter 112 provides the RF input signal to be amplified on input line 136 to the RF power amplifier 102.
The generation of the envelope signal and the I and Q baseband signals is known to one skilled in the art. Various techniques for the generation of such signals may be implemented. In FIG. 1 signal generation block 122 generally denotes the generation of these signals.
As known in the art, the path which the envelope signal follows from the digital-to-analogue converter 126a to generation of the supply voltage on line 138 to the power amplifier 102 suffers from delays and attenuation which vary on a unit by unit basis within a production tolerance. As also known in the art the path which the baseband signals follow from the digital-to-analogue converters 126b and 126c to generation of the RF input signal to be amplified on line 136 suffers from delays and attenuation.
In general, such delays and attenuation need to be controlled so as to ensure that they fall within certain tolerances, usually smaller than the production tolerances, to ensure maximum operating efficiency of the power amplifier and to ensure certain spectral emissions requirements are met (such as a minimum distortion of the amplified output signal). To achieve this, the signal processing in the envelope path prior to magnitude calculation must match that through the input (RF) path accurately and precisely. Further, the relationship between the amplitude of the signal in the envelope path and the amplitude of the signal in the input path must be correctly aligned.
In the envelope path delays may be introduced by several stages, such as the filter 128a, the output filter 106, and the supply feed 104. In addition, as denoted by an internal delay block 110 of the ET modulator 108, delays may arise in the ET modulator 108 itself. It should be noted that block 110 is illustrative of delays suffered in the ET modulator 108, and is not representative of specific circuitry or functionality of the ET modulator 108, which is not shown as it is not relevant to the present invention.
In the RF path delays may also be introduced by several stages, such as the respective I and Q filters 128b and 128c, and in the inter-stage SAW filter 112.
Amplitude errors may be introduced in an amplifier stage 150 of the envelope tracking modulator, the amplitude stage 134 of the input path, and in the supply feed network 104 to the amplifier. The filters 126b and 126c in the input path, and the filter 126a in the envelope path, are also sources of amplitude errors.
It is an aim of the present invention to provide an improved technique for controlling the relative delay and amplitude between the RF and envelope paths.